Change default push refSpec to 'refs/*:refs/*' (non-forced push)

Using forced push as implicit default for replication to slaves
could lead to unintended disruption on remote repositories.
Default git push refSpec is non-forced so the user would expect
Gerrit replication to apply the same default behaviour.

See: http://bit.ly/1jEU2vp for disruption caused by default forced push
Change-Id: I4509d3500c730b8b5921dc155ea0476a70b3de9e